1. Field of the Invention
The present invention relates to an image reading apparatus for separating stacked documents, conveying them one by one, and reading a document.
2. Description of the Related Art
There exist image reading apparatuses, such as a scanner and a scanner portion used in an image forming apparatus (e.g., a copier or a facsimile machine). One known example of an image reading apparatus of this kind includes a supporting portion for supporting sheet documents stacked thereon, a document conveying device for separating and conveying stacked documents one by one, and a flatbed scanner for reading a document conveyed one by one by the document conveying device. The document conveying device includes a document pickup portion that has a pickup roller for drawing one or documents from the supporting portion and a separating portion for separating the documents drawn by the pickup roller one by one. The flatbed scanner includes a document pressing plate for pressing a document against a reading surface and fixing it (hereinafter referred to as a pressing plate). The pressing plate can be freely closed and opened so as to be movable to a close position where it presses a document against the reading surface and to an open position where it withdraws from the reading surface.
Among such image reading apparatuses, one type of an image reading apparatus in which a document conveying device is disposed on a pressing plate suffers from impacts or vibrations on a document pickup portion occurring in an operation for opening or closing the pressing plate. An impact on the document pickup portion leads to a change in the position of the pickup roller, thus making it difficult to set documents on a supporting portion. One example approach to preventing this problem is a structure disclosed in Japanese Patent Laid-Open No. 2005-277533. This structure includes a sensor for detecting an operating of opening or closing a pressing plate and executes initialization of returning a pickup roller of a document pickup portion to an initial position upon detection of the operation of opening or closing the pressing plate by the sensor. However, the sensor for detecting opening/closing of the pressing plate in the structure described in Japanese Patent Laid-Open No. 2005-277533 leads to an increase in the cost of manufacturing the image reading apparatus.
Additionally, the structure described in Japanese Patent Laid-Open No. 2005-277533 does not detect opening or closing of the pressing plate when an output from the sensor for detecting opening or closing the pressing plate is not detected in a power-saving mode to which the image reading apparatus is switched from a normal mode. Therefore, if the pressing plate is opened or closed in the power-saving mode, initialization for the pickup roller is not triggered, so the initialization is not executed.
Similarly, if the pickup roller of the document pickup portion is changed by its own weight when the image reading apparatus is in a stand-by state for a long time while closing the pressing plate, the initialization for the pickup roller is not triggered, so the initialization is not executed.